In conventionally used microwave assisted magnetic heads, a coercive force of a spin injection layer of a spin torque oscillator is set below a gap field in order to achieve good recording performance without depending on polarity of recoding current.
To increase recording density further, reduction of a write gap (WG) and increase of microwave field strength are required. That is, greater driving current is required. However, if the driving current alone is increased while the spin injection layer is formed of a material conventionally used, stable oscillation becomes difficult to achieve.